Various types of display systems are known. An exemplary type of display system is a liquid crystal display. Two exemplary types of liquid crystal displays are twisted nematic liquid crystal displays and variable birefringence liquid crystal displays. In each of these two liquid crystal displays an optical output is produced in response to incident or input light and as a function of liquid crystal orientation, which Sometimes is referred to as liquid crystal structure, director, or molecular alignment.
In such liquid crystal displays, liquid crystal material is contained between a pair of parallel, spaced-apart substrates to form a liquid crystal cell. Electrodes or some other means are used to provide electric field to the liquid crystal material or to particular parts of the liquid crystal material to affect orientation and, therefore, optical effect. The input may be other than electric field. Examples include magnetic field and thermal energy inputs.
Surface treatment of a substrate tends to cause that liquid crystal material which is generally in proximity to the particular substrate to align in a preferred direction. Examples of surface treatment include rubbing the surface with cotton, felt, or some other material in a particular direction, which causes the liquid crystal material to align relative to that direction. Another example of surface treatment includes applying a silicon oxide (SiO) or silicon dioxide (SiO.sub.2) coating to the surface using an evaporation technique; depending on the angle of evaporation relative to the substrate surface and other factors, which are known in the art, the liquid crystal material will tend to align in a particular direction, which usually is generally parallel to the surface but at some tilt angle, as is well known. Other examples of surface treatment includes the applying of a polyvinyl alcohol (PVA) material to the surface or a polyimide coating to the surface. The various coatings also may be rubbed using cotton, felt or some other material to provide the desired alignment characteristics.
In a twisted nematic liquid crystal cell/the rub direction or primary alignment direction (not considering the tilt angle) at one substrate is at an angle other than 0 degrees or 180 degrees relative to the rub direction or primary alignment direction of the other substrate. For example, the rub or alignment directions of respective substrates may be at 90 degrees to each other in the standard twisted nematic liquid crystal cell so as to provide for a helical twist of 90 degrees in the liquid crystal alignment direction from one substrate surface to the other. In an example of a variable birefringence liquid crystal cell, the rub direction or primary alignment direction of one substrate usually is oriented in parallel with the rub direction or primary alignment direction of the other substrate. The mentioned parallel alignment may be at 0 degrees or at 180 degrees, and there may be the same or different tilt angles at or near respective substrate surfaces of the cell. Other relative alignment directions may be used in respective liquid crystal cells, as is known.
The optical effect may be a change in direction of plane of polarization of plane polarized light (sometimes referred to as linearly polarized light), change in characteristics of plane polarized light to elliptically or circularly polarized light, or vice versa, etc. The optical effect may be a retardation of one quadrature component of polarized light relative to the other quadrature component. Using appropriate optical analyzers, such as a plane polarizer, circular polarizer, wave plates, filters, etc., light from the display can be discriminated or demodulated thereby to form an optical output, such as an image, which is a function of the energization effected on the liquid crystal material at various portions in the liquid crystal display. The image may be information, e.g., alphanumeric type, a scene, view, picture, cartoon, etc., as is well known.
Although the invention is described below with respect to particular types of liquid crystal cells and displays, it will be appreciated that features of the invention may be employed with other types of liquid crystal displays and also with other types of displays, light shutters, and other optical output devices.
Various techniques have been used to provide input, such as an electric field, to a liquid crystal display. Electric field may be applied to selected areas of a display to determine whether or not a particular area is to affect light in a particular way or not. Such areas sometimes are referred to as pixels, pixel areas, picture elements, pels, etc. The assemblage of pixels may be used to form a particular image intended to be produced by the display for direct viewing, projection, etc.
Various driving techniques, i.e., techniques to apply electric field to selected areas of a liquid crystal display, are known. One driving technique is that using crossed electrodes in a matrix form. Another driving technique, especially when the liquid crystal device is used as a shutter, is to use a pair of electrodes respectively over the entire face or substantial portion of the face of each of the substrates forming the liquid crystal cell device or display. Alternatively, for a shutter having numerous electrodes, the electrodes or groups of them may be operated simultaneously selectively to cause desired optical effect over a relatively large area of incident light. Another driving technique is referred to as an active matrix driving technique (sometimes referred to as thin film transistor technique). To supply electrical power, control signals, reference voltage or potential, etc. to a liquid crystal cell of, such a display, usually there are one or more electrically conductive paths, sometimes referred to as traces or by some other name, at or on the substrates. Connections to such traces may be made by electrical connectors having contacts contained in or by a housing, by wire bonding techniques, and the like. The traces also may be or may, include terminal pads, which are electrically conductive and are exposed on a substrate especially for electrical connection thereto. Separate terminal pads also may be provided.
In an active matrix drive technique an active matrix or assemblage of transistors is formed on a semiconductor substrate. The active matrix may be sliced or skimmed from the substrate and applied, e.g., laminated, to a surface of a flat or smooth glass plate, for example. The plate then is used as one of the substrates for the liquid crystal cell. Typically the other substrate would be a transparent glass plate on the surface of which is formed a transparent electrode. The active matrix substrate may include for each pixel an electrode and a transistor. The transistor usually controls the signal to or connection of the electrode. Electrical connection lines or paths associated with the active matrix transistors may be coupled to an appropriate controller, such as a computer, a video receiver, etc., that selectively energizes respective transistors thereby to provide an appropriate signal on an electrode or electrodes controlled by such transistors and, accordingly, to produce an image. The electrode on the second-mentioned substrate may be coupled to a source of reference electrical potential, such as ground. Depending on whether a transistor is energized or not, and perhaps the extent or value of such energization, an electric field may be applied to the liquid crystal material between the active matrix electrode and the other electrode. The electrodes directly associated with the active matrix may be transparent if the display is a transparent one. Such electrodes may be other than transparent, for example, reflective, when the display is a reflective type.
The substrate on which the active matrix is formed actually may be a semiconductor substrate. An example of such a substrate and active matrix is disclosed in one or more of the above-mentioned patent applications, such as in Ser. No. 08/187,050.
A disadvantage of using an active matrix substrate or another semiconductor substrate in a liquid crystal display ,device is the special processing and procedures required to make a liquid cell or display which uses such a substrate. These procedures and processing usually add to the time and expense required to make a liquid crystal cell or display. Also, such special processing and procedures tend to decrease manufacturing yield of the devices, which increases cost per unit manufactured. It would be desirable to minimize the requirement for special processing and manufacturing of liquid crystal cells and displays which use such substrates.
Consistent with the foregoing, it would be desirable to use generally standard liquid crystal cell processing methods to make crystal cells and displays which use an active matrix substrate or a substrate made of semiconductor material. One disadvantage to obtaining alignment of liquid crystal structure in an active matrix liquid crystal cell by rubbing has been found to be damage which may occur to the active matrix materials due to the rubbing. It would be desirable to reduce the requirement for rubbing and, therefore, to increase yield and/or reliability of liquid crystal cells which use active matrix or semiconductor substrates.
Another disadvantage of an active matrix substrate used in a liquid crystal cell is the non-uniformity of the surface thereof, which usually has various peaks and valleys in the surface due to the electronic components formed therein. Such surface non-uniformity may have a noticeable degrading effect on the quality of images produced by a liquid crystal cell. For example, a change in path length of light in such a liquid crystal cell or a random alignment or misalignment of liquid crystal material due to a peak or a valley in a substrate may uncontrollably change optical phase retardation. This negative impact on the display is compounded if the display is used in a reflective mode because light then transmits through the liquid crystal twice.
Making electrical connections to the traces on a relatively rough or unsmooth surface of an active matrix substrate, whether laminated on glass or formed in a semiconductor substrate, may present difficulties compared to making connections to the traces on a smooth glass substrate. For example, due to the rough surface it is possible that the interaction with a contact of a connector may damage the contact, the trace, and/or the material of which the substrate is formed. It would be desirable to reduce the possibility of damage when electrical connections are made to a rough surface substrate.
Although it often is desirable to provide a large surface area of electrical connection to maximize the integrity of the connection, the relatively rough surface of the active matrix material may present a raised land of relatively small cross section that engages the connector contact, thus reducing the surface area of connection. It also would be desirable to maximize surface area of the electrical connections or otherwise to improve electrical connections to an active matrix display.
It would be desirable to simplify the making of electrical connections to the traces on the substrates of a liquid crystal cell. Examples of simplification could include making all of the electrical connections at only one of the substrates or by reducing the number of electrical connections that have to be made to one of the substrates. By reducing the number of electrical connections to one of the substrates, such as the active matrix substrate, and, thus, the density of those connections, and increasing the number of connections to the other substrate, the close spacing requirements for the connector contacts can be relaxed, and the difficulty of makring the connector and the accuracy required for placement of the connector in attached relation to the liquid crystal cell are reduced.
Homogeneous alignment usually refers to an alignment of liquid crystal material in a direction that is generally parallel to the plane of a surface of a substrate of a liquid crystal cell. The liquid crystal material may have a tilt angle relative to the surface. Various tilt angles are possible and are used in various types of liquid crystal devices, as is well known. Exemplary liquid crystal cells which use homogeneous alignment are twisted nematic liquid crystal cells, used, for example, in watch and computer displays. Homeotropic alignment of liquid crystal material usually refers to an alignment that is generally perpendicular (sometimes referred to as normal) to the surface or the plane of the surface of the substrate of the liquid crystal cell. In the past homeotropic alignment was-used in liquid crystal cells that work in a dynamic scattering mode in response to application of an electric current.
Nematic liquid crystal and smectic liquid crystal can have characteristics of birefringence, whereby the ordinary index of refraction and the extraordinary index of refraction are different. In a variable birefringence liquid crystal cell, by changing the orientation or alignment of the liquid crystal (or some of the liquid crystal) relative to the direction of light propagating through the liquid crystal, optical phase retardation can be varied correspondingly. Examples of variable birefringence liquid crystal cells in which optical phase retardation can be varied are described in U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, and 4,582,396, which are incorporated by reference. As is described in those patents, by changing the applied input, such as electric field, the alignment of liquid crystal material in the liquid crystal cells can be altered thereby to alter the effective optical phase retardation of the light transmitted through the liquid crystal material. As also is described in the just-mentioned patents, the liquid crystal material in proximity to the respective substrates has generally homogeneous alignment; these portions of the liquid crystal material or liquid crystal layer sometimes are referred to as the surface layers of the liquid crystal material and it is these layers or at least parts thereof which switch alignment in response to applied field input during operation of the liquid crystal cell to change the optical phase characteristics of the liquid crystal cell in response to say application or removal of electric field. The surface layers or surface portions are separated by a portion of the liquid crystal material or a layer thereof which generally is aligned with respect to the surfaces. Such perpendicularly aligned liquid crystal tends not to contribute to optical phase retardation (or whatever contribution it has relatively minimal compared to the possible phase retardation provided by the surface portions). Such generally perpendicularly aligned liquid crystal material also may tend separate the physical/mechanical interaction of the two surface portions of liquid material during operation of the liquid crystal cell as the surface portions switch from one alignment to the other. The liquid crystal material which tends to separate surface portions sometimes is referred to as the "bulk" liquid crystal; whether the bulk is more or less quantity of liquid crystal than the surface portions does not deter use of such label "bulk". Various means may be used to align the bulk portion of the crystal material. Those means may be electrical, mechanical, a combination or some other means, for example, as is described in the aforementioned patents.
It would be desirable to provide variable optical phase retardation in a reflective liquid crystal cell and display using such a cell, and, to do so in a liquid crystal cell that has an active matrix type substrate. It would desirable to provide substantial uniformity of operation and optical phase retardation characteristics in a variable birefringence liquid crystal cell while reducing the affect of and/or without regard to disparities in cell thickness due to peaks and valleys in the substrate.
A miniature image-source can be used to form an image for viewing. Such viewing may be directly, e.g., by the eye of a person, camera, etc. Sometimes such direct viewing requires one or more lenses to enlarge or otherwise to or to enhance the image produced by the image source. The viewing may be indirect by projecting the image from the image source onto a screen, for example, which is viewed. In the present invention a miniature image source uses one or liquid crystal cells (one being described in the embodiment below), and/or as shutters, or other types of optical display or shutter devices, to create images, etc. for direct viewing, for projection; etc. Additional optics, such as polarizers, analyzers, wave plates, optical retarders, filters, lenses, etc., and a light source also may be used with the miniature image source to create images. Power and control also may be provided, such as by electrical, magnetic, and/or thermal sources.